docs: def _prep_non_framed(self):
          """Prepare the opening data for a non-framed message."""
          self._unframed_body_iv, self.body_length = deserialize_non_framed_values(
              stream=self.source_stream, header=self._header, verifier=self.verifier
          )
  
          if self.config.max_body_length is not None and self.body_length > self.config.max_body_length:
              raise CustomMaximumValueExceeded(
                  "Non-framed message content length found larger than custom value: {found:d} > {custom:d}".format(
                      found=self.body_length, custom=self.config.max_body_length
                  )
              )
  
          self.__unframed_bytes_read += self._header.algorithm.iv_len
          self.__unframed_bytes_read += 8  # encrypted content length field
          self._body_start = self.__unframed_bytes_read
          self._body_end = self._body_start + self.body_length